Title: The formation of peak rings in large impact craters

Authors: Joanna Morgan1, Sean Gulick2, Timothy Bralower3, Elise Chenot4, Gail Christeson2, Philippe

Claeys5, Charles Cockell6, Gareth S. Collins1, Marco J. L. Coolen7, Ludovic Ferrière8, Catalina Gebhardt9,

Kazuhisa Goto10, Heather Jones3, David A. Kring11, Erwan Le Ber12, Johanna Lofi13, Xiao Long14,

Christopher Lowery2, Claire Mellett15, Rubén Ocampo-Torres16, Gordon R. Osinski17, 18, Ligia Perez-

Cruz19, Annemarie Pickersgill20, Michael Pölchau21, Auriol Rae1, Cornelia Rasmussen22, Mario

Rebolledo-Vieyra23, Ulrich Riller24, Honami Sato25, Douglas R. Schmitt26, Jan Smit27, Sonia Tikoo28,

Naotaka Tomioka29, Jaime Urrutia-Fucugauchi19, Michael Whalen30, Axel Wittmann31, Kosei

Yamaguchi32, William Zylberman17,33.

Affiliations:

1) Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK.

2) Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Texas 78758-

4445, USA.

3) Pennsylvania State University, University Park, Pennsylvania 16802, USA.

4) Biogéosciences Laboratory, UMR 6282 CNRS, Université de Bourgogne-Franche Comté, Dijon

21000, France.

5) Analytical, Environmental and Geo-Chemistry , Vrije Universiteit Brussel, Pleinlaan 2,Brussels 1050,

Belgium.

6) Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, EH9 3FD, UK.

7) Department of Chemistry, Curtin University, Bentley, WA 6102, Australia.

8) Natural History Museum, Burgring 7, 1010 Vienna, Austria.

9) Alfred Wegener Institute Helmholtz Centre of Polar and Marine Research, Bremerhaven, 27568,

Germany.

10) Tohoku University, International Research Institute of Disaster Science, Aoba 468-1 E303, Sendai

980-0845, Japan.

11) Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA.

12) Department of Geology, University of Leicester, Leicester, LE1 7RH, UK.

13) Géosciences Montpellier, Université de Montpellier, 34095 Montpellier Cedex05, France.

14) China University of Geosciences (Wuhan), School of Earth Sciences, Planetary Science Institute, 388

Lumo Rd. Hongshan Dist., China.

15) BGS, The Lyell Centre, Research Avenue South, Edinburgh, EH14 4AP, UK.

16) Groupe de Physico-Chimie de l´Atmosphère, ICPEES, UMR 7515 Université de Strasbourg/CNRS 1

rue Blessig, 67000 Strasbourg, France.

17) University of Western Ontario, Centre for Planetary Science and Exploration and Dept. Earth

Sciences, London, ON, N6A 5B7, Canada.

18) University of Western Ontario, Dept. Physics and Astronomy, London, ON, N6A 5B7, Canada.

19) Instituto de Geofísica, Universidad Nacional Autónoma de México, Cd. Universitaria, Coyoacán

Ciudad de México, C. P. 04510, México.

20) School of Geographical & Earth Sciences, University of Glasgow, Gregory, Lilybank Gardens,

Glasgow, G12 8QQ, UK.

21) University of Freiburg, Geology, Albertstraße 23b, Freiburg, 79104, Germany.

22) University of Utah, Department of Geology and Geophysics, 115 S 1460 E (FASB), Salt Lake City,

UT 84112, USA.

23) Unidad de Ciencias del Agua, Centro de Investigación, Científica de Yucatán, A.C., Cancún,

Quintana Roo, C.P. 77500, México.

24) Institut für Geologie, Universität Hamburg, Bundesstrasse 55, Hamburg, 20146, Germany.

25) Japan Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho, Yokosuka-city,

Kanagawa, 237-0061, Japan.

26) Department of Physics, University of Alberta, Edmonton, Alberta, T6G 2E1,Canada.

27) Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences FALW de Boelelaan 1085,

Amsterdam, 1018HV, Netherlands.

28) Rutgers University New Brunswick, Earth and Planetary Sciences, Piscataway Township, NJ 08854,

USA.

29) Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology,

200 Monobe Otsu, Nankoku, Kochi, 783-8502 Japan.

30) University of Alaska Fairbanks, Geosciences, 900 Yukon Dr., Fairbanks, AK 99775, USA.

31) Arizona State University, LeRoy Eyring Center for Solid State Science, Physical Sciences ,Tempe,

AZ 85287-1704, USA.

32) Department of Chemistry, Tohu University, Ota-ku Funabashi, Chiba 274-8510, Japan, and NASA

Astrobiological Institute.

33) Aix Marseille Univ, CNRS, IRD, Coll France, CEREGE, Aix-en-Provence, France. 

One sentence summary: Drilling reveals the lithology and physical state of the Chicxulub peak-

ring rocks, and confirms the dynamic collapse model for peak-ring formation.

Abstract:

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks

with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater

formation. As crater size increases, central peaks transition to peak rings. Without samples,

debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is

the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only

accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found

was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are

cross-cut by dikes and shear zones and have an unusually low density and seismic velocity.

Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Main text:

Impacts of asteroids and comets play a major role in planetary evolution by fracturing upper-

crustal lithologies, excavating and ejecting material from the impact site, producing melt pools,

and uplifting and exposing sub-surface rocks. The uplift of material during impact cratering

rejuvenates planetary surfaces with deeper material. Complex impact craters on rocky planetary

bodies possess a central peak or a ring of peaks internal to the crater rim, and the craters with

these features are termed central-peak and peak-ring craters, respectively (1). Most known peak-

ring craters occur on planetary bodies other than Earth, prohibiting assessment of their physical

state and depth of origin. Here, we address the question of how peak rings are formed using

geophysical data, numerical simulations, and samples of the Chicxulub peak ring obtained in a

joint drilling expedition by the International Ocean Discovery Program (IODP) and International

Continental Scientific Drilling Program (ICDP).

Upon impact, a transient cavity is initially formed which then collapses to produce a final crater

that is both shallower and wider than the transient cavity (1). Dynamic uplift of rocks during the

collapse of the transient cavity in the early stages of crater formation (Fig. 1B-C) likely forms

central peaks (2). The dynamic collapse model of peak-ring formation attributes the origin of

peak rings to the collapse of over-heightened central peaks (3). The observational evidence for

this model is most obvious on Venus, where central peaks gradually evolve into peak rings with

increasing crater size (4). The peak-ring to crater-rim diameter ratio increases with crater size on

Venus, but does not get much larger than approximately 0.5. The lack of any further increase in

this ratio led to the suggestion that in larger craters the outward collapse of peak ring material is

halted when it meets the collapsing transient cavity rim (4).

A different concept for peak-ring formation, the nested melt-cavity hypothesis, evolved from

observations of peak-ring craters on the Moon and Mercury (5-7). This alternative hypothesis

envisions that the uppermost central uplift is melted during impact, and an attenuated central

uplift remains below the impact melt sheet and does not overshoot the crater floor during the

modification stage. Hence, in contrast to the dynamic collapse model (Fig. 1), this nested melt-

cavity hypothesis would not predict outward thrusting of uplifted rocks above the collapsed

transient cavity rim material. The origin and shock state of rocks that form a peak ring are less

clear in the nested melt-cavity hypothesis, as they have not been evaluated with numerical

simulations. Head (6), however, postulated that material in the outer margin of the melt cavity

forms the peak ring and, therefore should be close to melting. This requires shock pressures of

just below 60 GPa. In contrast, Baker et al. (7) propose that peak rings are formed from

inwardly-slumped rotated blocks of transient cavity rim material originating at shallow depths

and, thus, should have experienced lower average shock pressures than simulated in the dynamic

collapse model.

The transition from central-peak to peak-ring craters with increasing crater size scales inversely

with gravity (1), suggesting the same transition diameter of about 30 km found on Venus (4)

should also hold for Earth, and that craters > 30 km in diameter should possess a peak ring.

Craters on Earth often display internal ring-like structures, but complications and uncertainties

due to target heterogeneity, erosion and sedimentation, make it difficult to distinguish peak rings

that are genetically linked to their extraterrestrial counterparts (8-9). Seismic reflection data

across the ~200-km diameter Chicxulub multi-ring impact structure revealed it to be the only

terrestrial crater with an unequivocal and intact peak ring, with the same morphological structure

as peak-ring craters on Venus, Mercury, the Moon, and other rocky bodies (10-14). These

seismic data and previous drilling also revealed a terrace zone formed from slump blocks of

Mesozoic sedimentary rocks, with the innermost blocks lying directly underneath or close to the

outer edge of the peak ring (Fig. 1G). This observation supported the idea that peak rings in

larger craters could be created through the interaction of two collapse regimes with the peak-ring

rocks being formed from uplifted crustal basement that had collapsed outward and been

emplaced above the collapsed transient cavity rim (15).

Numerical shock-physics simulations (e.g. Fig. 1) are consistent with the dynamic collapse

model in that they reproduce this mode of peak-ring formation as well as other crater features

such as the observed mantle uplift and terrace zone (16-20). For the simulation in Figure 1 we

used well and geophysical data to construct the pre-impact target, which is comprised of a 33-km

thick crust with ~3-km of sedimentary rocks above basement (21). We tracked the material that

eventually forms the Chicxulub peak ring and show that it originates from mid-crustal basement

(8-10 km depth) that is shocked to pressures over 10 GPa (Fig. 1A). The peak-ring rocks first

move outward and upward as the initial transient cavity forms (Fig. 1B), then progress inward to

form part of the zone of central uplift (Fig. 1C), and finally collapse outward to be emplaced

above collapsed transient cavity rim material composed of sedimentary rocks (light grey) that

were originally between 0- and 3-km depth (Fig. 1D-F). The dynamic collapse model therefore

predicts that the Chicxulub peak ring is formed from uplifted crystalline basement rocks.

Structural data from a number of exposed terrestrial impact structures supports the idea of

dynamic collapse of the central uplift (9, 22-24) and that, in larger craters, the peak ring is

formed from the interaction of two collapse regimes (25). In the simulation, the final crater (Fig.

1F) has the same key features as the upper 10 km of the Chicxulub crater as imaged on a radial,

depth-converted seismic reflection profile (21) (Fig. 1G). Specifically, a suite of dipping

reflectors mark the boundary between Mesozoic sedimentary rocks and peak-ring rocks, with

evidence of discrete melt zones within the peak ring (especially in the upper few hundred

meters).

Prior to drilling, not all of the geophysical data appeared to be consistent with the hypothesis that

the Chicxulub peak ring was formed from uplifted crustal basement. Gravity models and seismic

refraction data indicated that the peak-ring rocks had a relatively low density (2.2-2.3 g cm-3)

(26) and seismic P-wave velocity (3.9-4.5 km s-1) (27). The seismic velocity of crustal basement

rock outside the central crater is > 5.6 km s-1 (28), making it difficult to explain how crustal

rocks within the peak ring could have such a strongly reduced seismic velocity. The inferred

physical properties have been explained by the peak ring either being formed from a thickened

section of allochthonous impact breccia (26) that is a typical cover of crater floors or from

megabreccia (allochthonous breccia with large clasts > 10 m) as seen in one of the annular rings

at the Popigai impact structure in Siberia (8).

In April-May 2016, a joint effort by the IODP and ICDP drilled the Chicxulub peak ring offshore

during Expedition 364 at site M0077A (21.45° N, 89.95° W) (Fig. 2A). The drill site is located

at ~45.6 km radial distance, using a previously selected nominal center for the Chicxulub

structure of 21.30° N, 89.54° W (10). We recovered core between 505.7 and 1334.7 meters

below the sea floor (mbsf). We made visual descriptions through a transparent liner, while

samples from the end of the core barrel (core catcher) were available for direct inspection. We

made 114 smear slides and 51 thin sections from the core-catcher samples, which were taken at

regular intervals throughout the drill core. We measured petrophysical properties at the surface

using a Multi Sensor Core Logger (MSCL), and acquired a suite of wireline logging data from

the sea floor to the bottom of the hole (21).

The upper part of the cored section from 505 to 618 mbsf consists of a sequence of hemipelagic

and pelagic Paleogene sediments. We reached the top of the peak ring at 618 mbsf (Fig. 2A, B).

The uppermost peak ring is composed of ~130 m of breccia with impact melt fragments that

overlie clast-poor impact melt rock (Fig. 2B). We encountered felsic basement rocks between

748 and 1334.7 mbsf that were intruded by pre-impact mafic and felsic igneous dikes as well as

impact-generated dikes. We recovered one particularly thick impact breccia and impact melt

rock sequence between 1250 and 1316 mbsf. The entire section of felsic basement exhibits

impact-induced deformation on multiple scales. There are many fractures (Fig. 3A), foliated

shear zones (Fig. 3B) and cataclasites (Fig. 3C), as well as signs of localized hydrothermal

alteration (Fig. 3D). The felsic basement is predominantly a coarse-grained, roughly-

equigranular granitic rock (Fig. 3E), that is locally aplitic or pegmatitic, and in a few cases

syenitic. The basement rocks in the peak ring differ from basement in near-by drill holes

encountered immediately below the Mesozoic sedimentary rocks, suggesting a source of origin

that was deeper than 3 km (21).

In total 18 of the smear slides and 17 of the thin sections were prepared from the felsic basement

and viewed using an optical microscope. Evidence of shock metamorphism is pervasive

throughout the entire basement with quartz crystals displaying up to four sets of decorated planar

deformation features (Fig. 3F). We observed shatter cone fragments in pre-impact dikes between

1129 and 1162 mbsf, as well as within the breccia (Fig. 3G). Jointly, the observed shock

metamorphic features suggest that the peak ring rocks were subjected to shock pressures of

approximately 10 to 35 GPa (29). No clear systematic variation in shock metamorphism was

observed with depth. We note that impact melt, which is formed at shock pressures of >60 GPa,

is also a component of the peak ring (Fig. 2B).

The formation of the Chicxulub peak ring from felsic basement (Fig. 2) confirms that crustal

rocks lie directly above Mesozoic sedimentary rocks (Fig. 1G), which is consistent with the

dynamic collapse model of peak-ring formation (Fig. 1A-F). On the contrary, the nested melt-

cavity hypothesis does not predict this juxtaposition of units at the peak ring. In the numerical

simulation shown in Figure 1, the majority of the rocks that form the peak ring are subjected to

peak-shock pressures in the 10-35 GPa range, with some zones of melt rock (colored red), which

is also consistent with our drill-core observations. Conversely, in the nested melt-cavity

hypothesis, the peak-ring rocks are expected to be subjected to either higher (6) or lower average

shock pressures (7) than we observed.

We investigated the physical properties of the peak-ring rocks using: 1) core-based MSCL

natural gamma ray (NGR) and gamma density logs; and 2) downhole sonic logs and vertical

seismic profile (VSP) data that determine P-wave velocity surrounding the borehole (Fig.

2C)(21). The drilling data confirm that the peak-ring rocks have low densities and seismic

velocities, as suggested by geophysical models (26-27). The density of the felsic basement varies

between 2.10 and 2.55 g cm-3, with a mean of 2.41 g cm-3, and P-wave velocities vary between

3.5 and 4.5 km s-1, with a mean of 4.1 km s-1. These values are unusually low for felsic

basement, which typically has densities of > 2.6 g cm-3 and seismic velocities of > 5.5 km s-1.

We found that samples of the peak ring were variable in strength, locally quite hard or friable.

We also observed distinct variations in rate of drill bit penetration over short distances (< 1 m),

with some sections seeming mechanically weaker than others. Fracturing, shock metamorphism,

and other factors, such as hydrothermal alteration, may contribute to the reduction in seismic

velocity and density of the felsic basement. Dilation during brittle deformation is observed in

central uplifts in other large terrestrial impact craters (30-31), and dilatancy is predicted to

increase fracture porosity in the peak-ring rocks by between 1 and 5% (32). Shock

metamorphism can also reduce density, as shown in experiments (33) and in nature (34).

One of the most enigmatic and enduring fundamental unknowns in impact cratering is how bowl-

shaped transient cavities collapse to form larger, relatively-flat, final craters (1). To do so

requires a temporary reduction in cohesive strength and internal friction (35-36). In the model

shown in Figure 1, the rocks in the peak ring have moved a large distance (> 20 km) during

crater formation, hence, these units may well provide clues to the transient weakening

mechanism that allows large craters to collapse.

The confirmation of the dynamic collapse model (Fig. 1A-F) by the Expedition 364 results

provides predictions about shock-deformation, density reduction, and the kinematics of peak-ring

formation. These predictions can be tested and refined through drill-core investigations of

physical properties, paleomagnetism, structural data, and shock metamorphism. We anticipate

improvement in constraints on impact energy and the sizes of the transient and excavation

cavities. As a consequence, the volumes of environmental pollutants released by the K-Pg impact

will be better constrained, together with its role in causing the end-Cretaceous mass extinction

(37). As the deep subsurface biosphere is influenced by fracturing and mineralogical changes in

host rocks induced by shock and post-impact hydrothermal activity, understanding how impact

craters are formed and modify the environment will advance our understanding of deep

subsurface life on Earth and potential habitability elsewhere.

The validation of the dynamic collapse model also strengthens confidence in simulations of

large-crater formation on other planetary bodies. These simulations suggest that, as crater size

increases, the rocks that form peak rings originate from increasingly deeper depths (38). This

relationship means that the composition of the peak-ring lithology provides information on the

crustal composition and layering of planetary bodies, and be used to verify formation models,

such as for the Moon (e.g., 6, 38-39). One of the principal observations used to support a version

of the nested melt-cavity hypothesis in Baker et al. (7) is that peak rings within basins of all sizes

on the Moon contain abundant crystalline anorthosite and must, therefore, originate from the

upper crust if indeed the lower crust is noritic. Our results suggest a deeper origin for peak-ring

rocks, and thus are more in accordance with alternative models for the composition of a

heterogeneous lunar crust in which an anorthositic layer extends regionally to deeper depths (40-

41). The dynamic collapse model and Expedition 364 results predict density reduction by shock

and shear fracturing within the uplifted material (33), which is consistent with the recent Gravity

Recovery and Interior Laboratory (GRAIL) mission results of a highly porous lunar crust (42)

and the presence of mid-crustal rocks juxtaposed by shear zones in the peak ring at Schrödinger

crater (38). This linkage between deformation and overturning of material at the scales > 10 km

implies that over an extended period of time impact cratering greatly increases the porosity of the

subsurface and cause vertical fluxes of materials within the crust.

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Acknowledgments: This research used samples and data provided by the International Ocean

Discovery Program. Samples can be requested at: http://web.iodp.tamu.edu/sdrm/ after the end

of the moratorium on 19th October 2017. Expedition 364 was jointly funded by the European

Consortium for Ocean Research Drilling (ECORD) and the International Continental Scientific

Program, with contributions and logistical support from the Yucatan State Government and

Universidad Nacional Autónoma de México (UNAM).

Fig. 1. Dynamic collapse model of peak-ring formation. (A-F) Numerical simulation of the

formation of Chicxulub (18) at: 0, 1, 3, 4, 5, and 10 minutes tracking the material that eventually

forms the peak ring (indicated by the arrow in A), and records the maximum shock pressure

(blue color scale) to which the peak-ring rocks were exposed during passage of the shock wave.

The red color indicates zones of impact melt, for which shock pressures have exceeded 60 GPa.

The pre-impact target rocks are composed of sediments (light gray), crust (medium gray), and

mantle (dark gray). (G) Depth-converted, time-migrated seismic profile ChicxR3 (13). ChicxR3

is a radial profile (roughly west-northwest) that passes ~200 m from the location of M0077A.

For comparison with the simulation, shading is added to match the final crater shown in (F), with

light gray for inward-collapsed sedimentary rock, dark gray for peak-ring material, and white for

Cenozoic sedimentary cover (21). Black dashed lines indicate dipping reflectors at the outer

boundary of the peak ring and red dashed lines mark reflectors that may be consistent with zones

of impact melt rock in (F). IODP/ICDP Site M0077A is shown in (G) and placed in a similar

position on the magnified inset of the model in (F). VE = vertical exaggeration.

Fig. 2. IODP/ICDP Expedition 364. (A) Location of Site M0077A on depth-converted seismic

reflection profile ChicxR3 (13,14), overlain by seismic P-wave velocity (27). (B) Lithology

encountered at Site M0077A from 600 m to total depth, with Paleogene sediments (gray), breccia

with impact melt fragments (blue), impact melt rock (green), felsic basement (pink), and pre-

impact dikes (yellow). In order to indicate a possible difference in origin, the blue and green

color within the breccia is slightly lighter than in the dikes. (C) Corresponding petrophysical

properties: gamma density (g/cc) and natural gamma ray (NGR)(cps) measured on the cores

using an MSCL, and seismic P-wave velocity (km/s) obtained from sonic (red) and VSP (blue)

wireline logging data (21).

Fig. 3. Photographs from Expedition 364. Felsic basement displaying: (A) brittle faulting at

749.5 mbsf; (B) a foliated shear zone at 963.5 mbsf; (C) a cataclastic shear zone at 957.4 mbsf;

(D) hydrothermal alteration at 930 mbsf; and (E) typical granitic basement with large crystals of

red/brown potassium feldspar at 862.3 mbsf. (F) shocked quartz from 826.9 mbsf in cross-

polarised light, displaying three sets of planar deformation features (indicated by the solid white

bars). (G) Shatter cone fragment from an amphibolite clast in the breccia at 708.5 mbsf.

Supplementary Materials:

Material and Methods

Table S1

References 43-63.

Figure 1

T = 0 mins

Future peak-ring

T = 1 mins

T = 4 minsT = 3 mins

T = 5 mins T = 10 mins

A)

6080 40 20 0

6080 40 20 0

B)

C)

E)

G)

Peak ringMesozoic sedimentary rock

Depth

(km

)

Crystalline basement

Cenozoic sedimentary rock

20

10

0

-10

-20

-30

-40

20

10

0

-10

-20

-30

-40

20

10

0

-10

-20

-30

-40

0

1

2

3

4

5

6

7

8

9

10

Depth

(km

)

Radial distance (km)

Radial distance (km)

D)

F)

M0077A

0 12 24 36 48 60

6080 40 20 0

8090 70 60 50 40

7500 mProfile Chicx-R3

VE: ~2.5X

M0077A

Peak-shock pressurewithin peak ring (GPa)

1300

1250

1200

1150

1100

1050

1000

950

900

850

800

750

700

650

600

550

De

pth

(m

bsf)

Basement

A)

b)

Figure 2

M0077A

Densityg/cc

NGRcps

Seismicvelocity

km/s

C)

Impactbrecciadikes

Impact meltrock

Impact meltrock dikes

Pre-impactdikes

Felsicbasement

0.2

NW SE

0.6

1.0

1.4

Velocity (km/s)2.0

49.5 40.5

K-Pgboundary

K-Pgboundary

Peak ring

Post-impactsedimentaryrocks

Depth

(km

)

Depth

(km

)

Depth

(km

)

5.0

1.8 2.8 0 200 2.5 5.0

B)

1.0

0.6

0.7

0.8

0.9

1.1

1.2

1.3

1.0

0.6

0.7

0.8

0.9

1.1

1.2

1.3

Radial distance (km)

Pgsediments

Breccia withfragments ofimpact meltrock

B)A)

C) D) E)

G)F)

5 cm

2 cm

5 cm 5 cm

1 cm

50 µm

Figure 3

2 cm